Cooking of cellulose pulp in a cooking liquor containing pre-evaporated black liquor

ABSTRACT

The method is for cooking of cellulose pulp in which the raw material, preferably in the form of chips, undergoes a successive elevation of temperature towards the cooking temperature while the chips are first impregnated with an impregnation liquor followed by cooking in a cooking liquor that is preferably alkali. Through the addition of pre-evaporated black liquor, either as early as the impregnation stage or during the subsequent cooking stage, an improved delignification process is achieved in which the pulp obtains improved properties, primarily with respect to strength/viscosity for a given degree of delignification, and also with respect to yield. The method can be used both for continuous cooking and for batch cooking of cellulose pulp.

PRIOR APPLICATION

This application is a U.S. national phase application based onInternational Application No. PCT/SE03/00066, filed Jan. 17, 2003,claiming priority from Swedish Patent Application No. 0200189-9, filedJan. 24, 2002.

TECHNICAL FIELD

The present invention concerns a method for the production of celluloseby cooking cellulose pulp in a cooking liquor containing pre-evaporatedblack liquor.

THE PRIOR ART

The technology of cooking has undergone significant development forseveral decades. During the 1960s and 1970s, systems were used in whichessentially all liquor for cooking, including white liquor, were addedin batches at the pre-treatment stage. Cooking in continuous digesterssubsequently took place in the same liquor down through the digester,and the liquor was then withdrawn. Relatively high levels of alkali wereestablished at the start of the cooking stage, such that sufficientalkalinity was be maintained throughout the complete cooking stage. Itbecame clear that the high levels of alkali at the beginning of thecooking stage were detrimental to the quality of the pulp, and this ledto several variations on this system being suggested. These variationsinclude such techniques as MCC (modified continuous cooking), ITC(isothermal cooking) using the same cooking temperature throughout thedigester, and EMCC (extended modified cooking).

Later, during the 1980s, black liquor impregnation, in which liquor usedin the cooking stage is reintroduced to constitute part of theimpregnation liquor, was developed for both batchwise cooking and forcontinuous cooking. In this case, white liquor could be added batchwiseat the end of the impregnation, or at the beginning of the cookingstage. The used cooking liquor, i.e. the black liquor, that waswithdrawn from the cooking stage could in this case have a relativelyhigh residual alkali content, on which this withdrawn black liquor wasreintroduced to the impregnation. Here, most of the residual alkali wasconsumed before the used impregnation liquor was sent to recoveryfollowing withdrawal (concentration by evaporation and soda recoveryfurnace). This technique allowed a lower level of alkali to beestablished at the beginning of the cooking stage.

Several solutions are known in which black liquor is used as animpregnation liquor in an impregnation zone before cooking. A system isrevealed in U.S. Pat. No. 5,080,755 that has black liquor in the input.A variant is revealed in U.S. Pat. No. 5,053,108 in which black liquorwithdrawn from the digester is recycled to the high-pressure layer inorder to there form the major part of the treatment liquor in thetransfer circulation for the digester. A variant that has been developedfurther is revealed in EP477059, in which wood chips impregnated withblack liquor are raised to cooking temperature before the main additionof white liquor. These show that many different suggestions for processhave been studied, with the aim of improving the quality of the pulpwhile at the same time maintaining the high degree of delignification inthe pulp that is washed after cooking.

Marketing by Andritz-Ahlstrom of another cooking technology, denoted byLO-SOLIDS, began during the 1990s. This involves the continuouswithdrawal of cooking liquor with a high degree of organic material andthe replacement of this by new or treated cooking liquor that has alower level of dissolved organic material (DOM/dissolved organicmaterial). However, this system suffers from the disadvantage that thecooking process often results in a lower yield, caused by the fact thatwith the withdrawn organic material, such as lignin, also containscarbohydrates, principally hemicellulose.

Other methods of improving the cooking stage, both with respect to yieldand with respect to pulp quality, are the addition of polysulphide, AQor dissolved Xylan.

The yield is highly significant during cooking since an increase of only1% means that a production facility of normal size, having an output of1,500 tonnes a day, would experience an increase in production of 15tonnes, which, with a pulp price of 700 USD/ADT, gives an increasedincome of 10,500 USD a day.

THE AIM AND PURPOSE OF THE INVENTION

Subject to the developments in cooking technology, principally thetechnology used in continuous cooking, two dominating technologies arecurrently available. These are the technique known as “LO-SOLIDS” andthe technique developed by Kvaerner Pulping AB known as “COMPACTCOOKING”. Extremely high liquor/wood ratios are established duringCOMPACT COOKING in the initial phases of the cooking stage, with a veryhigh level of black liquor present in the cooking liquor.

It has now become clear, surprisingly, that a very favourabledelignification is established during the cooking stage if black liquoris present in the cooking liquor. This contrasts strongly with theprinciples of the LO-SOLIDS technology. Thus, cooking liquor with a highlevel of dissolved organic material is not withdrawn, and replaced withcooking liquor with a lower level of dissolved organic material, as itis in the LO-SOLIDS technology.

The problem is rather the reverse: it is desired to increase thefraction of dissolved organic material while at the same timemaintaining the levels of other added liquors, white liquor, etc., withrespect to amount and concentration. It is particularly desired toenrich the cooking liquor with the organic material that advantageouslyinfluences delignification and that also contributes, to a certainextent, to an increased yield.

The relevant type of desired supplement for the cooking liquor isalready available at the pulp mills, but it is present in theevaporation stage before the recovery. Nobody has yet realised that apartially evaporated black liquor has an advantageous effect on thecooking stage, nor have they realised that the partially evaporatedblack liquor should be returned to the cooking stage from theevaporation stage.

The main aim of the invention is to increase the selectivity in thecooking stage by accelerating delignification. This results either inthe achievement of improved pulp quality (viscosity/pulp strength) andhigher yield at the same degree of delignification (reduction in kappavalue), or in the achievement of a higher degree of delignification atthe same pulp strength and yield.

The invention also allows a positive increase in OH⁻ and in HS⁻ ionsduring the cooking stage, something that in a known manner gives betterselectivity and bleachability for the cellulose pulp produced.

Another purpose is the exchange of the improved delignification effectduring the cooking stage for at least one of the following advantages:

-   -   Smaller continuous digesters, that is: cheaper systems for a        given production volume (shorter stoppage times);    -   Increased production capacity for both batchwise and for        continuous cooking, with a maintained pulp quality;    -   Reduced need for cooking chemicals (alkalis), something that        gives lower production costs.

The invention can be used on both steam-phase digesters and on hydraulicdigesters; with inverted top separators, with downward-feeding topseparators and with types that lack a top separator; and it can be usedduring the production of cellulose pulp using both the sulphite processand the sulphate process. In the same way, deciduous wood, coniferouswood, annuals (such as bagasse, etc.) and others can constitute thesource of cellulose. The invention can be used with batchwise cooking,in which the chips are fed into a vessel in which a sequential treatmentwith various impregnation liquors and cooking liquors subsequently takesplace on the chips that are held stationary in the vessel.

The invention can also be used in continuous digesters, in whichpre-evaporated black liquor is added to cooking liquors that pass eitherupstream or downstream with the chips during the cooking stage, at thebeginning of the cooking stage, in the middle, or at its end.

DESCRIPTION OF DRAWING

FIG. 1 shows the principles of application of the invention in a cookingsystem.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows schematically a cooking system with the associated recoveryof cooking chemicals. The chips are first handled in a chip managementsystem 1 (Chip) in which the chips can preferably be made basic withsteam for the expulsion of air and for the first warming of the chips. Afirst addition of the process liquid occurs here such that the chipsbecome mixed into a chip/liquid mixture.

Black liquor BL1, which has been withdrawn from a subsequent cookingstage, is added at a subsequent impregnation stage (BL-Imp). The blackliquor can be added with the chips at the start of the impregnationstage and it can accompany the chips in what is known as downstreamtreatment, after which the black liquor is withdrawn from theimpregnation vessel. This black liquor BL2 normally has a relatively lowlevel of alkali of about 5-20 g/l, and it can, after a pressurereduction in a cyclone/pressure reduction vessel FL, be sent forrecovery. The pressure is normally reduced down to a pressure level thatdoes not exceed an excess pressure of 0.5 bar, and often to a pressurethat is essentially atmospheric pressure. The cooking process takesplace at an elevated pressure, normally with an excess pressure of 5-25bar in the digester, and a preceding impregnation with black liquor caneither take place under pressure or at atmospheric pressure.

The treated chips after impregnation with black liquor are transferredto the digester (Cook) where cooking liquor Cliq is added. The chips arecooked during the cooking stage at a cooking temperature that lies inthe interval 150±20° C., after which the cellulose pulp is transferredfor further delignification and bleaching (Bleach), preferably passingthrough an intermediate washing stage (not shown in FIG. 1).

Recovery consists, in a conventional manner, of a number of evaporationstages 5 a-5 e in which the black liquor, which is at essentially zeroexcess pressure, (BL2 after passing through FL) is subject to anevaporation in several stages from an initial level of dry mattercontent TS in the black liquor (BL2) of 17-20%, to a level that liesover 70-80%. The evaporation line 5 a-5 e consists of a number ofevaporation stages, know as effects, that the black liquor passesthrough in sequence, while heating steam is passed in the oppositedirection to the flow of black liquor. Normally, the earliest andhottest steam is used in the evaporation stage that treats the blackliquor with the greatest level of dry material, that is, the final stageseen from the point of view of the flow of black liquor. Expelledvolatile substances are also obtained at each evaporation stage, andthese are dealt with by special gas management systems (not shown inFIG. 1) or by turpentine recovery systems, or they are led to otherstages in order there to be mixed with the is heating agent (the steam).

Superconcentrators 6 may be included as a last stage of the evaporation,before the black liquor is combusted in a soda recovery furnace 7. Amelt is formed in this furnace that is removed from the bottom (as shownin FIG. 1), and that is suspended to form green liquor (not shown) andsent to a causticization plant where white liquor is reformed.

Naturally, in contrast to what is shown in FIG. 1, the first evaporationstage can be constituted by a specially designed evaporation stage thatonly evaporates that amount of black liquor that is to be recycled tothe impregnation stage or the cooking stage. Such a stage can thus belocated next to the digester and does not necessarily need to bearranged in association with the other evaporation stages prior to thesoda recovery furnace.

According to the invention, a portion of the partially evaporated blackliquor PV_BL is thus removed from, for example, the first stage 5 a andled back to the cooking stage. As is shown schematically in the figure,the pre-evaporated black liquor PV_BL can be added to a digestercirculation in which cooking liquor is withdrawn from the cooking stage,normally through strainers in the wall of a continuous digester, and isthen returned to the centre of the digester through a central pipeplaced at the same height as the strainers. In this way, thepre-evaporated black liquor will become mixed with other treatmentliquor before it is added to the cellulose material before the latter iscooked at the actual delignification stage. The pre-evaporated blackliquor can, in such an addition process, be added at a location in thedigester at which the bulk delignification stage starts.

The pre-evaporated black liquor can, in one alternative, be added to thecooking liquors CLiq that are to be added to the digester before thecooking stage. This alternative is shown using dashed lines in FIG. 1.In this way, the pre-evaporated black liquor can be mixed with othercooking liquors before addition to the digester, and at such an earlystage that the pre-evaporated black liquor constitutes part of thecooking liquor at the initial delignification stage of the cookingstage.

The invention can be modified in a number of ways within the frameworkof the claims.

For example, the invention can also be used during batchwise cooking ofchips that have been filled into the vessel, following the sequence:

-   1) Filling the vessel with chips-   2) Heating the chips with steam-   3) Heating/impregnation with warm black liquor-   4) Heating/impregnation with hot black liquor-   5) Cooking with cooking liquor-   6) Cleaning compression following the cooking stage, in which the    expelled cooking liquor is stored in tanks for hot black liquor-   7) Cleaning compression following the previous stages with cleaning    liquor, where the liquid expelled first is stored in tanks for warm    black liquor.

Emptying of the cooked and washed chips.

In this type of cooking sequence, the pre-evaporated black liquor can beadded in batches to the cooking liquor that is added to the chips instep 5 above. Alternatively, a modified impregnation stage according tostep 4 above can be used, in which the pre-evaporated black liquor isadded in batches to the hot black liquor added in step 4, alternativelythe purging of the vessel with pre-evaporated black liquor as aconclusion of step 4, in order to expel residual amounts of hot blackliquor that have not been enriched with pre-evaporated black liquor.

As a further alternative, the pre-evaporated black liquor can be addedin batches to the cooking stage during the commencement of step 5,whereby the pre-evaporated black liquor is included into a digestercirculation for mixture with the cooking liquor that remains incirculation in the vessel during the cooking stage.

The pre-evaporated black liquor consists, according to the invention, ofa heat-treated black liquor that has a content of dry matter (TS) thatexceeds the level of dry matter that can be obtained in the black liquorthat is withdrawn from the process and whose pressure is subsequentlyreduced. This is equivalent to the black liquor that is denoted by BL2in FIG. 1 and whose pressure is reduced in at least one pressurereduction vessel, FL in FIG. 1. The content of dry matter TS in thisliquor can normally lie around 17-20%, and the content of dry matter TSof the pre-evaporated black liquor is, according to the invention, to beraised by at least 10% from this level, to 27-30%.

An improved effect of the delignification process is obtained even atthis modest increase in the content of dry matter. The black liquor ispreferably evaporated further to a content of dry matter of at least30-40%, and preferably at least 50%.

The higher the content of dry matter, the less the optimal liquor/woodratio will be affected during the cooking stage without having to reducethe necessary batchwise addition of other cooking liquors.

The black liquor that, according to the invention, is pre-evaporated canbe constituted by pressurised black liquor that has been directlywithdrawn from the digester or indirectly withdrawn through a blackliquor impregnation, which may be either under pressure or essentiallyat atmospheric pressure, and subsequently pre-evaporated. The blackliquor may also be constituted by such black liquor that has passedthrough a reboiler, in which the black liquor is first used to generatesteam before being pre-evaporated in the manner according to theinvention.

Twice as much recirculated pre-evaporated black liquor with a content ofdry matter of 27-30% is normally required than would be required if thecontent of dry matter was around 60%, given similar conditions in thedigester with respect to other parameters.

The amounts that are available for return to the cooking stage dependon:

-   -   the current content of dry matter in the pre-evaporated black        liquor    -   the current liquid/wood ratio during the cooking stage    -   the amount of black liquor that accompanies the chips from a        preceding black liquor impregnation    -   the current raw material (deciduous wood, coniferous wood,        annuals, eucalyptus, etc)    -   the required batchwise addition of white liquor and its        concentration of alkali    -   the other types of cooking chemicals that are added.

When liquid/wood ratios that lie at the lower end of the range, around3-3.5:1, i.e. 3 to 3.5 m³ liquid for each cubic metre of chips, theamount of pre-evaporated black liquor with a content of dry matteraround 40% should exceed at least 5% of the total liquid amount, whichcorresponds to an amount of pre-evaporated black liquor of around0.15-0.175 m³ for each cubic metre of chips, in order for an appreciableeffect of the delignification to be achieved. At higher liquid/woodratios during the cooking stage, from around 7:1 and up towards 8:1, anequivalent increase of the minimum amount of pre-evaporated black liquorthat may, at the most, be required is around 0.35-0.40 m³ per cubicmetre of chips, in order to achieve an appreciable effect on thedelignification. If the increased liquid/wood ratio in the cooking zoneis established by internal recirculation of the cooking liquor, therequired increase for an effect on the delignification will be smaller.

Thus, relatively modest amounts of pre-evaporated recirculated blackliquor are sufficient that in normal cases amount to a few percent ofthe total amount of liquor in the cooking stage.

As much as 20-40% of the total amount of liquor can be constituted bypre-evaporated black liquor in cooking situations in which the cookingprocess has been established with a very low fraction of residual blackliquor from the impregnation.

The invention, however, is not uniquely determined by the amounts thatare recycled since, as has been previously mentioned, other processparameters during the cooking stage can influence the amount required,as can the type of cellulose pulp (deciduous wood, coniferous wood,annuals, etc.) that is being cooked.

The invention is based on the principle of retuming, in contrast toother cooking methods, dissolved organic material to the cookingprocess, which released organic material has been enriched by initiallyundergoing substantial evaporation with the purpose of increasing thecontent of dry matter in the black liquor. Thus, the concentration ofthe organic material increases, which has surprisingly turned out toinfluence the delignification process in an advantageous manner and tocontribute to increased yield, principally due to the hemicellulose inthe pre-evaporated black liquor being reprecipitated onto the cellulosefibres.

It is important for obtaining the best effect that the pre-evaporatedblack liquor is present during the main part of the bulk delignificationstage, more than 50% of the retention time of the chips in the bulkdelignification stage, and preferably during the initial delignificationstage.

The recycled pre-evaporated liquor can be further heat-treated in aseparate stage and/or certain fractions can be mechanically separated,and it can be adjusted with respect to the levels of other chemicals.

1. A method for the production of cellulose pulp, comprising: adding animpregnation liquor to an impregnation stage; treating wood chips in theimpregnation stage with the impregnation liquor at an impregnationtemperature; cooking the wood chips in a cooking stage by using acooking liquor having a cooking temperature of 150±20° C., the cookingtemperature being higher than the impregnation temperature; withdrawingblack liquor from the impregnation stage, the withdrawn black liquorbeing at the impregnation temperature and having a first concentrationof dry matter (TS); subjecting the withdrawn black liquor to a pressurereduction in a pressure-reduction vessel to form a pressure-reducedblack liquor having a second concentration of dry matter (TS), thesecond concentration being higher than the first concentration;conveying the pressure-reduced black liquor to an evaporation stage; inthe evaporation stage, forming a partially evaporated black liquor, thepartially evaporated black liquor having a third concentration of drymatter (TS), the third concentration having a dry matter (TS)concentration that is at least 10% higher than a dry matter (TS)concentration of the second concentration of the pressure-reduced blackliquor; adding the partially evaporated black liquor to the cookingliquor at a start of or during a delignification stage; and cooking thewood chips in the cooking stage to produce cellulose pulp.
 2. The methodaccording to claim 1, wherein the partially evaporated black liquor hasa concentration of dry matter (TS) in the partially evaporated blackliquor that exceeds 30%.
 3. The method according to claim 2, wherein thepartially evaporated black liquor has a concentration of dry matter (TS)in the partially evaporated black liquor that exceeds 50%.
 4. The methodaccording to claim 1, wherein the partially evaporated black liquor ismixed with another treatment liquor before the partially evaporatedblack liquor is added to the wood chips prior to the wood chips beingcooked in a delignification stage.
 5. The method according to claim 1wherein the method further comprises reducing a pressure of thewithdrawn black liquor in at least one pressure-reduction tank down to apressure that does not exceed an excess pressure of 0.5 bar andevaporating the pressure-reduced black liquor in at least one primaryevaporation step in which the evaporated black liquor is heated during aremoval of volatile substances from the evaporated black liquor toincrease a content of dry material.
 6. The method according to claim 5,wherein the method further comprises making the primary evaporation steppart of a recovery plant for black liquor.
 7. The method according toclaim 5, wherein the primary evaporation stage is constituted by adedicated evaporation stage that only evaporates an amount of blackliquor that is to be returned to the impregnation or cooking stage. 8.The method according to claim 1 wherein the partially evaporated blackliquor is added to the cooking liquor so that the partially evaporatedblack liquor is present during at least part of an initialdelignification stage of a cooking stage.
 9. The method according toclaim 1 wherein the partially evaporated black liquor is present duringmore than 50% of a retention time of the wood chips in a bulkdelignification stage.
 10. The method according to claim 1 wherein thethird concentration has a concentration of dry matter (TS) in thepartially evaporated black liquor that is in a range of 27-30%.
 11. Themethod according to claim 1 wherein the partially evaporated blackliquor is mixed with cooking liquor before being added to the start ofor during the delignification stage of the digester.
 12. The methodaccording to claim 1 wherein the method further comprises treating woodchips in several stages including impregnation and cooking stages atsuccessively increasing temperatures.